How to make your Perfect Dovetails - Spacer Method

The picture shows some dovetails made using the "Perfect Dovetails" method. This is actually the
joint at the front of a drawer from a chest of drawers. All the drawers were made
using the technique, which proved to both more accurate and quicker than traditional handcut
methods.

This page describes the 'spacer' method. Please note that this method will not
work unless your boards are exactly the same width with exactly square ends
and parallel sides. However, with that
proviso, it works pefectly well (and the joint shown here was made using it).

The materials required for this are very simple:

Some scrap hardwood and/or MDF.

Some coloured masking tape

...and, of course the wood for your joints

For tools, you need:

A bandsaw that tilts both ways (if yours doesn't tilt, you can make a jig - see Lonnie Bird's book or ​this website).

Router and table for lapped dovetails only (optional - otherwise some hand cutting is required)

The instructions provided here take you through the entire process of creating your perfect dovetails.
It is recommended that you read these through completely before starting so that you have a good
idea
how it all works.
After that, you can dip in and out as you need to. There is also a simplified instructions sheet to keep
handy in the workshop.
The general approach is illustrated by the picture of the bandsaw cutting the pins with the
bandsaw table tilted to the left. Note the labelling of the
spacers and the board.

Before you start

Accuracy is the secret to great dovetails, whatever method you
use. The "perfect dovetails" method relies on two important things
before you start:

an accurate cut from the bandsaw

accurately dimensioned boards

As regards the bandsaw, any decent
quality bandsaw should be up to the task so
long as it

has a two-way tilting table,

can be set
with the fence either side and

has been properly set up and
maintained

and

has a
good sharp blade in it.

With short
cuts in fairly thin pieces of timber, such
as for dovetails, blade wander is not such a
problem as it can be for deeper and longer
cuts, but nevertheless you do want to be as
accurate as possible and a smooth cut is
important, so use a new blade with, say,
4tpi for larger boards or 6tpi for smaller
ones.

As regards the boards to be joined, when using the spacer method, they must
be exactly the same width and each board must be
of an even thickness. (The importance of
having exactly equal width boards cannot be
underestimated. A 0.5mm difference between the board widths will translate to
a 0.5mm error in the relative sizes of pins and
tails. This is because the spacer method references off both edges of the boards). Fine tune
the widths to within 0.2mm of each
other by either planing or building up an edge with
strips of tape. The edges should all be
square and smooth. Don't forget to make the boards
fractionally longer than the finished length to
allow for trimming the joint after it is complete (a
typical trim allowance might be 0.5mm in which case allow an extra 1mm length if the
board
is jointed at both ends).
Also, if you are making
multiple joints then make sure that similar
boards are exactly the same dimensions as each
other. But you would do all that normally
anyway, right?

Having got the basics right, you now need to
know exactly the dimensions of your boards. I
advise working in millimeters throughout as they
are a useful scale for most dovetail joints and
the design tool takes decimal inputs, not
fractions, but if you want to work in cubits,
feel free.

You also need to know the kerf of
your bandsaw blade. This is quite critical, so
it is worth measuring it accurately as follows:

Make a board from scrap timber that has
smooth and parallel sides and is 4-6"
(100-150mm) wide. Measure this width
accurately (to the nearest 0.2mm) using a
vernier gauge. (See picture)

Make 4 or more lengthways cuts - i.e.
cutting into 5 or more pieces - as straight
and cleanly as possible.

If the sawn edges are rough you can clean up them up minimally to
remove the worst of the sawn roughness (e.g frayed edges which prevent the pieces fitting back
together), but
don't go overboard. It is better to use a really good blade which leaves a clean cut.

Now put the pieces back together and clamp them well, then
re-measure the width. The clamping of pieces is to simulate the tightness of fit of the required joint,
so no gaps should be apparent.

Subtract the new width from the old, divide this difference by 4, or however
many cuts you made and that is your saw kerf. (See picture)

Finally, before you start the design tool, label the edges of your boards "A "
and "B " so that they all match.
For drawers, I normally label the top edge "A ". (see picture above).

Design the dovetails and calculate the
spacers

Now it's time to plan your perfect dovetails and see what they will look like. Put all
your measurements into the "design tool" calculator form, namely:

The saw kerf you calculated earlier

The width of your two boards (which should be the same)

The thickness of each board (which may be different)

You need to decide how many tails you want, but you can always change this if you want
to. You also need to decide the dovetail angle to use.
I recommend using a whole number of degrees (which makes it easier to see on your bandsaw
table
tilt gauge). 8 degrees is about 1:7 which is a good all-purpose angle.
Put these details into the design tool calculator. By the way, it is perfectly possible to
use a
dovetail angle of zero to make a finger joint, but please note the comments in the
"Tweaks" section.

Now design and enter the pin widths. The dimensions in each case are the widths at the
base of the pins (the widest part). It makes life easier if you set this to be just a
fraction more than the
width of one of your chisels, so that chopping out the waste between the tails is
simpler (i.e. the width of a chisel plus a saw kerf is good).
One nice thing about the "perfect dovetails " method is that you can make
narrow pins in the traditional style, without being constrained by router shanks.
Make them as wide or narrow as you like, you can always change it in the design stage. Of
course, they will
need to be wide enough to allow the tails to be cut (i.e so that the narrow end of the pins
is a bit more than the saw kerf), but the calculator will let you
know if you go too narrow.

You also have to specify the widths of the half-pins at each edge of the board.
These will normally be a bit wider as you don't want them to break off when fitting
(they have nothing the other side to support them).
You may also need one of the half-pins to accommodate a groove for a drawer bottom.
Talking of drawers, you may want to have the back of the drawer a different width from
the sides
(to accommodate the bottom of the drawer running underneath the back, say, and/or to reduce
the top of the back to allow for
easy insertion of the drawer).
You can do this by specifying a cut-away for the pin board on either or both edges.
If you use my convention of the top being side "A" and the bottom being side "B", then you
will, for example, set the "Cut-away on Edge 'B'"
to be the distance from the bottom of the drawer sides to the top of the groove in them
which takes the bottom piece.
(Note that either the pin board or the tail board could be cut down after making the
joint, but it is usually the pin board that is affected).

Finally enter the "trim allowances ".

The "end trim allowance" (when positive) allows the tails
to protrude slightly so that they can be neatly trimmed. If you are making a lapped dovetail joint (see
later notes), then enter a negative end trim allowance equal to the amount of the covering thickness
required on the pin board.

The "fitting allowance" allows you to trim the cut sides of the tails and pins slightly without causing
gaps, giving a much more accurate finish to the joint. For "workmanlike" rather than "show" joints, a
fitting allowance of zero may be used and the joint should fit straight from the saw. Otherwise,
0.3mm will give an excellent finish without adding too much work (this equates to removing 0.15mm from
each side).

Now click "submit " and the calculator will draw your joint for you as well as
calculate the spacers that you will need.
You may get a number of warning messages if the calculator doesn't quite like your
specification. Hopefully these are self explanatory.
If it can draw the joint then it will do so and this might help you to understand any
error messages and how to fix them.

Don't forget to put the name of your joint in the box at the top of the calculator. If you are logged in
then all the details will be saved. Otherwise they will not be saved and you will
need to print them before navigating away from the page.

All about the spacers

Look at the "spacer widths " table in the calculator. You will see that there
are two columns, labelled "Edge A to fence " and "Edge B to fence ".
If the joint is symmetrical (i.e. the half-pins on each edge are the same width) then these
columns will be
identical, otherwise the last spacer will differ.
In either case, there are the same number of spacers as there are tails.

You will need to cut spacers of the specified width, ideally from a stable hardwood or
MDF. You will also need a jig for making the tails, cut from a similar wood.

To cut the jig use the pattern illustrated in the picture - make it about 10" (250mm)
long and with a diagonal section at the angle you have chosen for your tails and the "notch"
at right angles to this.
The dimensions are not critical, but the angle (and the right-angled notch) does need to be
accurate.

To make the spacers:

Make them from a similar thickness board as the jig and as long as the cut-out
diagonal section of the jig.

Cut them as accurately as you can (a table saw with a fine blade is best, but
otherwise finish with a plane and abrasive paper stuck on a flat surface).
In particular, ensure that the edges are smooth and parallel.
If the widths are very slightly off then this will result only in slightly different
sized tails
- the joint will fit so long as the spacers are numbered and always used in the same
order.

Except for the last spacer, fix a "cap" to the top of each spacer as shown. It is
easiest to fix one cap piece across them all and then cut to separate - see picture.

Write the number on each spacer - it is important to use them always in the same order,
in case of slight differences in width.

The last spacer (or pair of spacers if the edges are different) does not have a cap, but
has a notch cut at he same angle as the tails. There are two ways (at least) of doing
this.

Make a small "filler piece" (length specified by calculator). Place this in the
notch of the jig and place the spacer behind it (see picture),
adjusting the fence so that the blade just brushes the corner of the spacer. Now
remove the filler piece, move the spacer into the notch of the jig and cut the
notch in the spacer.

Or: Cut the notch in the corner of the spacer in a similar fashion but without using the
filler piece. Now
drill a 3.5mm hole in the top end of the spacer and insert an M4 bolt (waxed) and
screw in until it protrudes by the amount specified for the length of the filler
piece (see picture). The advantage of this method is that it allows for small
changes to improve the fit and also means that the spacer can be used for other
projects (which might require a different sized filler piece). Do make sure that the
bolt is close enough to the inside (flat) edge of the
spacer to fit behind the cap on the adjacent spacer.

Label the end spacer(s). If there are a pair of different spacers label them clearly
"Edge A towards fence" or "Edge B towards fence".

Write the width on the bottom ends of all spacers, so that they can be re-used in other
projects if required.

Prepare the boards

If you haven't already done so, label the boards clearly on the face side
(outside of
the box or drawer).
When using the spacer method, the face side will be uppermost throughout the bandsaw work, for both pins
and tails. Also make sure that the edges
are labelled "A" and "B" so that you can keep them aligned as you
work (and so that you use the right spacers, if the end spacers are different). I find
it best to put labels on masking tape as it is clearer and does not mark the wood.

Double-check that the boards are exactly the same width and that this is the same width that
you have
entered into the design tool.
Also double-check that the edges are parallel. (Measure twice, cut once!)

Mark the shoulders on each board, using a cutting gauge. For the tail boards, the width
of the shoulder must be the thickness of the pin board plus the trim allowance as
specified in the calculator. For the pin boards, the trim allowance is up to you. Mark the pin board on
both sides and mark the tail board all the way round. If you put coloured masking tape on before marking
the line, then peel it away, you will have a clear edge to cut to (see picture).

Do a test

Now, particularly if this is the first time you have used the "Perfect Dovetails"
method, I advise that you make a test joint.
Use two short pieces of wood the same thickness and width as your real boards and work
through the whole joint-making process.
(Label the board edges in the same way as the real boards). It's a good idea not to
remove all the waste, particularly at the edges of the tail board,
since then you can use your test pieces as a template to easily check your fence setting
when you cut the real boards. (see picture)

Note that on the test pieces in the picture I have done two joints - one for the front
and one for the back of a drawer.
The joint can be test fitted as shown without fully removing the waste on the tail board
edges.

Cutting the pins

Now at last you get to make the joint. We will cut the pins first. Place the bandsaw
fence to the left of the blade and tilt the table anti-clockwise (when viewed from your normal position -
i.e. fence lowest) by 8
degrees (or whatever angle you have chosen).
Place all the spacers against the fence (leave out the angled jig - that's only for
cutting the tails). Make sure the spacers are in the right sequence - lowest nearest the
fence.
They should all fit neatly in place, caps overlapping. The last (end) spacer has no cap
and should be placed so that the notch is not next to the blade. If there are two types
of end spacer, because the jont is asymmetric, then use the correct one - i.e. "Edge A
against fence" if the board you are going to cut will have edge A nearest the fence
(remember that you will be cutting the pin board with its face - labelled - side
uppermost).
Now adjust the fence so that the blade just touches the end spacer (a good test is to
run the blade in the wrong direction, by hand - if it lifts up the spacer then it is too
close). See picture.
Remove the end spacer.

Place the pin board, face side uppermost, with the correct side nearest the fence, into
the notch formed by the cap of the spacer (made available after removing the end spacer).
It is a good idea to leave the end spacer,
which you just removed, on the bandsaw table to remind you which edge should be nearest
the fence. Also, if you made a test joint, you can use this to check that the cut will
be in the right place.
Now slide the whole assembly forward (see picture), cutting down to just meet the shoulder
line (you can
use a stop if you wish, but I tend to do it by eye).
Remove the outer spacer and repeat for the next cut. Repeat until the last cut is with no
spacers.

Next, move the fence to the right of the blade and tilt the table the other way (clockwise) by your
chosen angle (again the fence will be lowest). Place the spacers as before (counting the
numbers from the fence) except that your end spacer may be different because now the
other edge of the board will be nearest the blade. Adjust the fence, remove the end
spacer and proceed as before (test as necessary with your test piece).

Cutting the tails

To cut the tails, place the table level and leave the fence on the right (assuming that
you just cut the pins, that's where it will be). Place your angle jig against the fence
and place the spacers (in sequence) against it. Use the appropriate end spacer (for
whichever side of the tail board will be nearest the fence) but this time place it so
that the notch is next to the blade. Adjust the fence so that the blade just touches the
notch, but does not rub against it (this is to ensure
that you do not make the joint too loose). See picture (move the assembly forward to
ensure the notch does not bang the blade).

Remove the end spacer and cut the tails similarly to the pins, removing one spacer at a
time. Always slide the whole assembly - in particular, do not just slide the board back after cutting
as it will jam and possibly damage the tail.

Move the fence to the left of the blade and repeat the procedure (but with the other end
spacer, if it is different).

Multiple joints

This section describes how to streamline the process when making, for example, a set of
drawers. This makes the production of set of drawers a relatively quick process - and
all with "Perfect Dovetails". Reading this section is optional, particularly
first time around. It is assumed that you know how to cut the pins and tails as
described in the earlier sections.

Let's assume you are making a set of drawers. You will probably have two types of joints
- one for the fronts and one for the backs, so you will need to do two calculation
sheets. You will therfore also have two sets of spacers (although you may have some
spacers the same size which are shared between the sets - see the tweaks section on how
to achieve this).

First cut the pins.
With fence left and table tilted anti-clockwise at the dovetail angle:

Cut all the back pin boards using the "back" spacer set (remembering to use the
correct end spacer depending on which edge is nearest the fence).

Cut all the front pin boards using the "front" spacer set.

With fence right and table tilted clockwise at the dovetail angle, repeat as above (but with the
other end spacers, as appropriate).

Now cut the tails.
With fence right and table level:

Cut (one side of) the back tails of all the drawer RHSs using the "back" spacer set
(this might be, say, edge A against the fence).

Cut (one side of) the front tails of all the drawer RHSs using the "front" spacer
set (this would then be edge B against the fence).

Repeat for the drawer LHSs, but note that the spacers for the other edge will be
needed in each case.

With fence left and table level:

Cut the back tails of all the drawer RHSs using the "back" spacer set (this might
be, say, edge B against the fence).

Cut the front tails of all the drawer RHSs using the "front" spacer set (this would
then be edge A against the fence).

Repeat for the drawer LHSs, but note that the spacers for the other edge will be
needed in each case.

You need a certain amount of presence of mind to do this. Avoid distractions. Label the
ends of the boards "Front" and "Back", depending on the type of joint. Then just
remember to check before you cut that (a) you are using the corrrect spacer set and (b)
you have used the correct end spacer, depending on which edge of the board will be
nearest the fence. It is a good idea to double check with your test piece before
cutting.

Removing waste and fitting the joints

Mark all the waste before cutting. The space between pins can be mostly cut out with a
bandsaw, if you wish, but do cut with the face side down so that you do not
inadvertently cut the pins themselves. Remove the remaining waste with a coping saw or
fretsaw, then with a chisel, as for hand-cut dovetails. See here for
an excellent guide on removing waste.

Test fit the joint. It will probably be a little tight in places, so carefully trim with
a sharp chisel as needed. Again, there are plenty of guides available on how best to do
this - for example this
one.

Drawers, and boards of unequal width

Typically drawer backs will be narrower than the sides, so that the bottom, running in
grooves in the sides, can go underneath the back.
Also the top of the back may be lower than the sides by a few mm, with the back end of
the sides feathered to match. This section describes how to produce these, including
cutting out the waste correctly.

As described in the opening section, start with the boards the same size. If this is
not possible, make up the narrow boards to the correct width using strips of wood
attached with glue or double-sided tape (they will be removed later).

Enter the amount of the cut-out for each edge in the calculator. Make sure that the
bottom half pin is wide enough to include the groove.
The groove will need to fall within a tail at the front, so do a calculator for that
too and check that it will work.

Make all the cuts as usual.

Do not remove the waste at the bottom edge of the drawer sides (tail boards).

Cut the groove on the insides of the drawer sides, using your router, plough plane
or whatever. N.B. if the ends of the front tails are to be exposed, make sure that
the groove is stopped short of the front.

Cut an exactly similar groove in the back pin board. (You will need a similar groove
in the front board too, so you may as well cut that at the same time).

On your bandsaw, cut the bottom off the back pin board, using the top edge of the
groove as a guide - i.e. cutting just to the waste side of the top of the groove.

Use the top of the groove to reference a mark around the shoulders of back end of
the tail board (i.e. the back end of the drawer side).

Mark the waste and remove. The resulting half pin on the back should then locate
above a "finger" containing the groove on the side.

The top of the back pin board is simply trimmed as required and the sides feathered
to match.

Lapped dovetails

The method described above is only designed to produce through dovetails that are, to
all intents and purposes like the hand-cut ones, just quicker and more consistent.
If you are making drawers then you may not want the dovetails to show on the face (on
the other hand you may be very happy to display your "Perfect Dovetails" in true "Arts
and Crafts" style).
In this case there are three alternative options:

Make an overlay drawer, using a separate overlay board on the front; or

Rebate the edges of the drawer front by at least as much as the thickness of the
sides and add a cockbead to give a flush drawer; or

Use lapped dovetails on the front.

If you are determined to use lapped dovetails then:

Plan the dovetails as usual, but enter a negative trim tolerance in the spreadsheet,
equal to the thickness to be left in the front board.

Cut the tails on the bandsaw as per instructions.

If you do not have a router with a table, then mark out the pins either:

in the traditional way, by laying the tail board on the pin board and
marking with a knife; or

using the spacers as if you were making a through dovetail, but only making
a very shallow cut to act as a guide for a dovetail saw.

If using method (b), do not cut deeper than the trim tolerance you have allowed for
the pins. Remember, this can be any amount you want - it
does not need to be the same as the tails trim tolerance, which in this case is
negative. So you could plan to trim 1mm from each end of a pin board, giving ample
to make a clear guiding cut. Also use a dovetail saw (in step 4 below) with a kerf as
close as
possible to, but no wider than, the bandsaw kerf.

Cut out the waste in the traditional way. Don't forget to mark the top edges of the
tails first and do not cut above this line,
especially if using method (b) above since the guiding cuts will extend above it

Cutting lapped dovetails with a router (jig-free!)

It is much quicker if you have a router (with the right bit) and table; the best way is:

Using a piece of scrap the same width as the real pin board and at
least as thick as the tails, cut the pins on the bandsaw as for a "through" dovetail joint. The tops
of the pins may get cut off, but don't worry (so long as they are as deep as the lapped joint
requires). Check the fit - you can clean up a little, but you want it to be tight.

Now use your scrap piece to set up your router bit and table. Use a moveable fence square to the
main fence. Set the main fence to the required length of cut and set the bit to the required depth.

Set the moveable square fence to the right of the bit (to ensure
that the bit rotation acts against the fence rather than away from it) and place your "test" piece
next to it, adjusting so that the bit just touches the inside of the required pin; clamp it in place
using a fence stop to secure against the force of the rotating bit (see picture).

Check that the bit does not quite cut the pin on the scrap board, then rout your real pin board. I
also suggest you check the depth and length of the cut on your scrap before routing the main piece.
I adjust the length of the cut so that it will undercut slightly at the back of the joint but stop
fractionally short on the inside fence, leaving a simple clean-up with a chisel

Repeat this process, setting the bit against each side of the "test" pins, then cutting the real
piece. If the tails are wide, you will also need to make cuts in the centre between pins.

Cut back to the shoulder and clean out the corners (or round over the insides of the tails,
according to your preference).

Fancy tails, re-using spacers and other tweaks

Once you are comfortable with using the "Perfect Dovetails" method and the "Design Tool",
you will realise that you can start to tweak the results to get some interesting and
useful results. Before discussing the main types of tweak, it is worth mentioning what
happens if you change the dovetail angle. You can do this quite easily by altering the
value in the calculator. It is easy to see how the new angle looks, so that you can
choose an angle which is aesthetically pleasing (but bear in mind the performance of the
wood).
You can also put in an angle of zero to make a finger joint. However, note that if you
do this, there is an important change to the spacers: The last spacer does not
require a notch and no angle jig is required. However, when cutting the tails, you need
to add a thin shim (thickness equal to the saw kerf plus the fitting allowance) in place of the angle
jig.

There are two possible types of tweaks you can apply:

Modify the spacer widths. This is useful if you wish to re-use spacers from another
joint. Bear in mind that this will change your pin widths.

Change the sizes of individual tails (and therefore the corresponding pins). This
allows you to make a huge variety of decorative joints. If you are doing lapped
dovetails, you can make the base of the tails just a fraction wider than a forstner
bit - to make removing the waste between the pins easier.

In each case the tweaks are applied interactively in the calculator and you can see how the
joint will look.

Modifying the spacer widths:
Click on "Tweak spacer widths" at the top of the table of spacer widths. Note that this
only lets you change the widths of the main group of spacers and then the end spacers
separately, so that the tails will still all be the same size. Put the new sizes into
the form. When you click "Apply", the main input form will be updated for the new pin
sizes as well as updating the spacer widths and the diagram of the joint. If you click
"Clear" then the tweak form will be cleared, but the input form is unchanged, so there
is no "Undo" function - you will need to re-input the original pin sizes or spacer
widths. Use the diagram to check your tweaks make sense (if they are really daft you
will get error or warning messages).

Modifying the tail sizes:
Click on "Tweak tail sizes" at the top of the table of pin and tail dimensions. This
operates slightly differently from the spacer widths tweak. Here you enter the amount by
which to increase or decrease each side of each tail. Click "Apply" to see the results.
Note that individual spacer widths will be changed to enable the joint to be made. To
clear all tweaks, click "Clear" then "Apply". Note that the tail tweaks are relative to
the main inputs and basic spacer widths - if you tweak the tails and then tweak the
spacer widths, the tail tweaks will still be applied relative to the new basic inputs.